Liquids produced in oilfield applications comprise a hydrocarbon component, (which may be a low density oil, termed condensate, or a medium density oil, termed medium oil or a high density oil, termed heavy oil) and some accompanying water which may be naturally occurring with the oil or may have been pumped into the reservoir to help drive out the oil. In order to process the oil successfully, the water must be separated out allowing relatively dry oil to be exported. In turn the water itself must be treated to an acceptable content of oil suitable for disposal.
Separation of water from a predominantly oil stream or treatment of a water stream to remove oil is generally more difficult and expensive as the droplet size of the dispersed, minority phase decreases. In many cases, there is a need to increase droplet size to improve separation or reduce costs of separation.
In most oilfield applications there is a need to remove gas from the liquid phase. This may be in the form of foam which needs to be broken down, discrete gas bubbles in a liquid which are required to be removed as part of a separation process, or dissolved gas which needs to be evolved from solution as discrete bubbles and removed as part of the separation process. Foaming is deleterious to the separation function since it may, for example, fill process equipment. It is commonly suppressed by the continuous use of additive chemicals. Furthermore, foaming can lead to false readings on apparatus such as level detectors in vessels. Therefore it is desirable to break down foam as quickly and cheaply as possible without the use of chemicals.
US Statutory Invention Registration No. H1568 discloses a coalescer which proposes the application of a standing wave ultrasonic field with a frequency range between 20 kHz and 1 MHz, with 680 kHz disclosed as an optimum frequency, for wastewater. The standing wave is created using at least two radially opposed pairs of transducers to cause coalescence of oil droplets in a flowing wastewater stream, with subsequent separation in conventional separators. The wastewater flows through a circular section vessel and the preferred embodiment includes seven pairs of transducers in groups at particular positions axially along and external to the treatment vessel. Intensity of application of acoustic energy is below cavitation levels.
However, the applicant's research has shown that the configuration disclosed in this document is unlikely to succeed as the fluid velocities typically encountered in pipeflow are too high and the flow is too turbulent for the ultrasonic forces to be effective. Also the residence in the field of the transducers is too short.
U.S. Pat. No. 5,527,460 described a multi-layered composite resonator system using a plane transducer and an opposing and parallel plane mirror for separating particles suspended in a fluid on a small scale. The technique uses an ultrasonic resonant wave.
Other prior art solutions include the use of electrostatic treatments which aim to use forces created by the interaction of electrically charged bodies to cause coalescence. For oil-from-water separation, filter-oalescers are commonly used to grow drop size by means of interference coalescence such as meshes or packing. Such techniques, in the case of electrostatic coalescers are not effective on water continuous-mixtures, or in the case of filter coalescers have limitations due to the likelihood of blockage.
U.S. Pat. No. 6,210,470 describes apparatus for degassing a moving liquid. The apparatus uses a transducer and reflector arrangement to produce ultrasonic standing waves which are inclined at an acute angle to a horizontal axis of liquid flow.
The techniques described below act to coalesce the dispersed droplets into larger droplets thereby improving the downstream separation efficiency and/or reducing the cost of downstream separation. In another embodiment the techniques described below are used to breakdown foam or promote the separation of suspended or dissolved gas, which also improves separation and/or reduces the costs of downstream separation.